This invention concerns a crystalliser for continuous casting as set forth in the main claim.
The crystalliser according to the invention is applied in the high speed continuous casting of billets and blooms of any type and section, and is used to obtain products of a high inner and surface quality.
In the state of the art of continuous casting, tubular crystallisers are used as an alternative to plate crystallisers, and consist of a substantially monolithic hollow body, the transverse section of which defines the section of the cast product.
To cool the molten metal cast inside the crystalliser, and hence to begin the progressive solidification of the metal, the state of the art provides a jacket outside the walls of the crystalliser which defines a transit compartment inside which a cooling liquid is made to pass.
This embodiment, which is widely known and used, has some disadvantages.
To guarantee an adequate structural rigidity of the crystalliser, also because crystallisers used at present are relatively limited in length, in order to prevent deformations due to the thermal and mechanical stresses during the casting process, the wall of the crystalliser must have a defined minimum thickness.
At present, crystallisers usually used have a length of less than 1000 mm and walls with a minimum thickness of around 13 mm, and in any case about 10% of the width of the billet or bloom cast.
As the casting speed increases, the thermal flow exchanged between the cooling liquid and the molten metal also consequently increases, and therefore the thermal flow which is transmitted through the walls of the crystalliser.
Moreover, because of the great thickness of the walls, there is a considerable difference between the temperature of the outer face and the temperature of the inner face of the crystalliser.
The thermal conditions which are created in the walls of the crystalliser considerably lower the mechanical properties, particularly the structural rigidity, of the material of which the crystalliser is made (copper or copper alloys), and this causes permanent deformations and distortions which create considerable technological problems and problems of quality in the cast product.
First of all, the deformations and distortions cause a modification to the inner taper along the crystalliser, which becomes progressively very different from the taper specified by the design plans, with the consequence that the inner cavity of the crystalliser no longer correctly follows the shrinkage of the solidifying skin.
This causes considerable problems in the quality of the cast product and it becomes necessary to reduce the casting speed. Moreover, the deformations and distortions can also cause a modification to the transverse section of the crystalliser, thus determining both surface and internal defects in the cast product.
Furthermore, the deformations and distortions shorten the working life of the crystalliser.
A further disadvantage which is particularly serious is that permanent deformations and distortions are generated in the area of the meniscus.
In fact, in this area uncontrolled interactions are created between the walls of the crystalliser and the skin which is forming; this causes the formation of deep oscillation marks on the surface of the cast product, an uncontrollable heat exchange, defects in the planarity of the skin, inner cracks in the areas near the corners which can cause risks of the skin breaking at the outlet of the crystalliser, and a leakage of the liquid metal.
Another serious disadvantage of monolithic crystallisers comes from the behavior of the cast product in correspondence with the corners. Since in this area the cooling acts on both sides, the skin tends to shrink in a differentiated manner, with the result that it is impossible to form the desired thickness and phenomena may occur such as the skin breaking at the outlet of the crystalliser.
In either case, the skin formed is not uniform and there are both surface and inner defects in the product.
The present Applicant has devised and embodied this invention to overcome these shortcomings and to obtain further advantages as will be shown hereafter.
The invention is set forth and characterised in the main claim, while the dependent claims describe other innovative characteristics of the invention.
The purpose of the invention is to achieve a crystalliser for continuous casting suitable to guarantee a great structural rigidity such as to eliminate the risks of permanent deformations and distortions even when there are extremely high heat stresses due to the intense heat exchange between the cooling liquid and the molten metal.
This structural rigidity is obtained without reducing the cooling capacity required for a correct solidification of the cast metal even at high casting speeds.
The crystalliser according to the invention has a monolithic tubular structure consisting of a wall with an outer face and an inner face in contact with the cast molten metal.
According to the invention, the crystalliser has through holes, made in the thickness of its wall, inside which the cooling liquid is made to circulate.
Therefore, the distance between the cooling liquid and the molten metal is reduced, yet without reducing the overall thickness of the wall of the crystalliser and therefore its mechanical and structural rigidity.
To be more exact, the holes are arranged so as to have their longitudinal axis at a distance of between 5 and 20 mm, advantageously between 7 and 15 mm, from the inner face of the crystalliser and therefore substantially from the liquid metal.
Thanks to the presence of the cooling liquid inside the wall of the crystalliser, it is possible to obtain a lower average temperature of the wall, thus reducing the heat stresses which lead to permanent deformations and distortions.
Moreover, there is a considerable reduction in the difference between the temperature of the face of the wall in contact with the cooling liquid and that of the inner face in contact with the molten metal.
All this allows to contain the deformations and distortions inside an elastic field, thus allowing to recover the original shape when the stresses are finished.
The crystalliser according to the invention is longer than 1000 mm, advantageously between 1050 and 1500 mm.
This increased length, together with the holes made directly in the monolithic structure of the crystalliser which allow to maintain the width of the wall at a certain value, gives great rigidity and resistance to mechanical and thermal stress.
The advantages which this solution according to the invention brings are, first of all, that the inner taper of the crystalliser remains according to specifications, and is therefore configured to follow the shrinkage of the cast product during solidification.
It is thus possible to maintain the high quality characteristics of the cast product, and to keep the casting speed high, thus obtaining high productivity.
Moreover, the possible causes of defects in the product are eliminated, such as lack of planarity, the presence of cracks near the corners, the formation of deep oscillation marks.
Furthermore, the working life of the crystalliser is extended.
The crystalliser according to the invention consists of a monolithic body of the tubular type, the inner cavity of which defines the section of the cast product.
According to a variant, the cooling in the corners of the crystalliser is controlled in a different manner from its plane zones.
This allows to appropriately condition the shrinkage of the cast product in correspondence with the corners, which shrinkage is faster than in the plane zones since the cooling acts simultaneously from two sides of the corner.
According to one embodiment of the invention, in correspondence with the corner, the cooling liquid does not flow through the holes made in the walls with the same volume and/or pressure as the liquid passing in the plane zones of the crystalliser.
According to a variant, the holes in correspondence with the corners are provided with a lower density with respect to the plane zones of the tubular wall of the crystalliser.
According to a further variant, the holes in correspondence with the corners are provided with a different shape, for example of a lesser section, with respect to the plane zones of the tubular wall of the crystalliser.
According to another characteristic of the invention, in correspondence with the corners, the wall of the crystalliser has reinforcement and stiffening inserts, or segments with a greater thickness, suitable to guarantee a greater rigidity in correspondence with the zones more subject to stresses, and also a lesser heat exchange.